Auto-Transformers

An auto-transformer has windings common to primary and secondary. So that the input and output circuits are electrically connected as one continuous winding per phase.

In the common part of the winding, the input and output currents are superposed.

The principal application of the auto-transformer is in case where voltage ratio is not great.

Applications: Boosters,static balancers and induction motor startors.

Adavntages:saving in copper, cores and losses

As compared to an ordinary 2-winding transformer of same output, an auto-transformer has higher efficiency but smaller size. Moreover, its voltage regulation is also superior.

Weight of copper in auto-transformer=(1-K)×Weight of copper in ordinary 2-winding transformer. Here K=V₂/V₁

Saving is increase when K incrasing unity

Inductively transferred power=(1-K)×input

Conductively transferred power=K×input

Rating of auto transformer = K/(K∼1)× Rating of 2-winding transformer

Practice questions:
1.A single-phase transformer has a rating of 15 k VA, 600/120V. It is reconnected as an auto- transformer to supply at 720 V from a 600 V primary source. The maximum load it can supply is
a. 90 kVA
b. 18 kVA
c. 15 kVA
d. 12 kVA

2.An auto-transformer having a transformation ratio of 0.8 supplies a load of 10 kW. The power transferred inductively from the primary to the secondary is
a. 10kW
b. 8kW
c. 2kW
d. Zero

3.In case of auto-transformers, which of the following statements are correct?
1. An auto-transformer requires less copper as compared to a conventional, 2-winding transformer of the same capacity.
2. An auto-transformer provides isolation between the primary and secondary windings.
3. An auto-transformer has less leakage reactance as compared to the conventional, 2-winding transformer of the same capacity.
Select the correct answer using .the codes given below:
a. 1, 2 and 3
b. 1 and 2
c. 1 and 3
d. 2 and 3

4.In an auto-transformer of voltage ratio V1 / V2 and V1 > V2, the fraction of power transferred inductively is
a.V1/(V1+V2 )

b.V2/V1

c.(V1-V2)/(V1+V2 )

d.(V1-V2)/V1

5.A single-phase, 10 kVA, 2000/200 V, 50 Hz transformer is connected to form an auto transformer as shown in the figure given above. What are the values of V1and I2 respectively?

a. 2200 V, 55 A
b. 2200 V, 45 A
c. 2000 V, 45 A
d. 1800 V, 45 A

6. What is the efficiency of an autotransformer in comparison to that of a two winding transformer of the same rating?
a. Slightly less than that of a two-winding transformer
b. Same as that of a two-winding transformer
c. More than that of a two-winding transformer
d. As low as 1/5th of the efficiency of a two-winding transformer

7. A two-winding transformer is converted into an auto-transformer. If we apply additive polarity and subtractive polarity for the connections, then the secondary voltage is 2640 V and 2160 V, respectively. What is the ratio of primary to secondary voltage on the original transformer?
a. 66 : 54
b. 54 : 66
c. 10 : 1
d. 1 : 10

Solved problems
1.A 5000-VA 480/120-V conventional transformer is to be used to supply power from a 600-V source to a 120-V load. Consider the transformer to be ideal, and assume that all insulation can handle 600 V.
(a) Sketch the transformer connection that will do the required job.
(b) Find the kilovoltampere(kVA) rating of the transformer in the configuration.
(c) Find the maximum primary and secondary currents under these conditions.
SOLUTION (a) For this configuration, the common winding must be the smaller of the two windings, and N_SE=4N_CThe transformer connection is shown below: